Replaced HashSet.UnionWith() with loop to avoid heap alloc

Author: louismclaughlin

com.unity.netcode.gameobjects/Runtime/Core/NetworkBehaviourUpdater.cs

@@ -34,7 +34,10 @@ internal void NetworkBehaviourUpdate(bool forceSend = false)
 #endif
             try
             {
-                m_DirtyNetworkObjects.UnionWith(m_PendingDirtyNetworkObjects);
+                foreach (var o in m_PendingDirtyNetworkObjects)
+                {
+                    m_DirtyNetworkObjects.Add(o);
+                }
                 m_PendingDirtyNetworkObjects.Clear();
 
                 // NetworkObject references can become null, when hidden or despawned. Once NUll, there is no point